The invention relates to a continuously variable transmission or CVT provided with an electro-hydraulic control system and a method for operation thereof. The CVT is known, for example, from EP-A-0.787.927 and is in particular suited for motor vehicles.
The known CVT comprises a V-type drive belt wound around a first pulley provided on a first shaft and a second pulley provided on a second shaft. Both pulleys have two conical discs, an axial separation of which is adjustable by means of a piston/cylinder assembly of each pulley. During operation of the transmission the drive belt is clamped with a first clamping force between the discs of the first pulley and with a second clamping force between the discs of the second pulley. The clamping forces are effected by exerting a pressure in a piston/cylinder-assembly associated with a pulley for each of said first and second pulleys. The level of the clamping forces determines the maximum amount of torque that can be transmitted between said first and second shaft virtually without relative movement in the tangential direction between the drive belt and the discs of the pulleys, i.e. belt slip, occurring. The (instantaneous) maximum amount of torque is referred to as the (instantaneous) torque transmitting capacity of the CVT. In the CVT a transmission ratio is related to the ratio of the running radii of the drive belt between the discs of the pulleys, which may be influenced by changing the ratio of the pressures in the piston/cylinder assemblies. An electro-hydraulic control system is provided to control said pressures and to achieve an appropriate transmission ratio and torque transmitting capacity of the continuously variable transmission.
It is generally known in the art, that the efficiency of the CVT is inversely proportional to the clamping forces applied by the pulley discs to the drive belt. Therefore, the control system is preferably arranged such that the lowest allowable clamping force for achieving torque transmission without belt slip is applied to the belt. For effecting the clamping forces, the control system is provided with a hydraulic system comprising a source of hydraulic medium in the form of a pump and a line pressure valve for controlling the pressure of the medium delivered by the pump to a main hydraulic line, the so called line pressure. To minimise energy loss, said line pressure is equal to, but not higher than, the highest pressure applied in the cylinders of the piston/cylinder assemblies. The control system is further provided with an electronic system comprising an electronic control unit (ECU) for generating a control current operating said line pressure valve in dependence on one or more variables, such as the transmission ratio, rotational speed of the pulleys and/or the torque to be transmitted by the transmission. Usually said line pressure valve is operated through applying a range of control pressures to said valve, which control pressure is regulated by an electronically operable pressure regulator, which in turn is operated by said control current. The pressure regulator derives said control pressure from an essentially constant pressure regulator feed pressure, which is somewhat higher than the maximum level of the control pressure to be applied. The control current generated by the ECU for operating the pressure regulator typically varies in a range between 0 and 1 Ampere. In dependence on said control current the control pressure regulated by the pressure regulator typically varies in a range between 0 and 6 bar, whereas the feed pressure is typically about 7.5 bar.
The known set-up of the control system is adopted both in conventional CVTs where line pressures are applied ranging from a few bar to 45 bar and in the currently favoured transmission designs, where the maximum line pressure level applied may range from 80 to 90 bar depending on CVT design specifics. Current developments in CVT design indicate that said maximum level might increase even further.
It appears that in the currently favoured CVT designs the accuracy with which the line pressure may be controlled does not meet the accuracy formerly achieved in conventional CVT designs. However, as was mentioned earlier, it is desirable for efficiency considerations to be able to accurately control the line pressure, so that the lowest allowable line pressure is set, while still effectively preventing belt slip.
It Is an object of the invention to improve the efficiency of currently favoured CVT designs and in general to provide for a CVT with a control system capable of accurate control of the line pressure. According to the invention these objects may be achieved by a CVT having the features defined in the characterising portion of claim 1. It is also an object of the invention to provide for a method for accurately and efficiently operating the CVT of currently favoured design. According to the invention this object may be achieved by adopting the method according to claim 13.
The CVT of the invention is characterized by pressure control means that are capable of influencing the line pressure by a further pressure acting on a part of the line pressure valve. In effect this means that the line pressure valve is controllable both by said control pressure and by said further pressure. This has the advantage that a number of options are opened for a considerable improvement of the accuracy of the control system and thereby of the transmission efficiency. According to the invention it is, for example, possible to use the control pressure to operate the line pressure valve so that approximately the required line pressure is set and to use the further pressure to fine tune the setting of the line pressure valve. In this manner it is achieved that the accuracy of the line pressures control and thus also the efficiency of the CVT is improved considerably.
According to an elaboration the invention the pressure control means comprise a further pressure regulator capable of regulating the further control pressure that serves as said further pressure. According to the invention it is advantageous, if in a first part of the range of applied line pressure levels, e.g. the lower halve between a few bar and approximately 45 bar, the line pressure valve is controlled by the control pressure regulated by the known pressure regulator, whereas the a second part of said range, e.g. the upper halve approximately between 45 and 87 bar, the valve is controlled by the further control pressure serving as said further pressure and regulated by the further pressure regulator. In this manner the pressure regulator and the further pressure regulator are effectively adopted in series, so that the total range of control pressure levels available for controlling the line pressure valve is essentially doubled. It was found that the extra cost from incorporating such pressure control means might be overcome by the positive effect they have on the efficiency of the CVT.
According to another elaboration of the invention, which is particularly suited for a CVT having a hydraulic circuit which is arranged such that the pressure in the cylinder of the piston/cylinder associated with the first pulley is said line pressure, whereas the pressure in the cylinder of the piston/cylinder associated with the second pulley is determined by means of a flow control valve capable of controlling a flow of hydraulic medium from said main hydraulic line to said piston/cylinder assembly or a flow from said piston/cylinder assembly to said reservoir. According to the invention the pressure control means comprise a passage connecting the piston/cylinder assembly associated with the second pulley to the line pressure valve, such that the pressure in said piston/cylinder assembly serves as said further pressure capable of influencing the line pressure. The control system, or more in particular the line pressure valve, is arranged such that when said pressure in the piston/cylinder assembly associated with the second pulley increases, the range of line pressure levels that can be set by regulating the control pressure decreases. Since the range of the control pressure levels remains unchanged, an increasingly more favourable ratio between the range of line pressure levels and the range of control pressure levels is obtained, when the pressure in the cylinder of the piston/cylinder assembly of the second pulley increases. The accuracy with which the control system can control the line pressure is thus made dependent on the pressure in the piston/cylinder assembly of the second pulley in such a manner that said accuracy increases when said latter pressure increases. Since the pressure in the piston/cylinder assembly of the second pulley increases with increasing transmission ratio, which for this purpose is defined as the rotational speed of the first pulley divided by that of the second pulley, the accuracy of the control system increases with increasing transmission ratio.
The advantage of such a control system is given by the insight that a CVT is operated for relatively long periods of time at the higher transmission ratios, the so-called OD-ratios. The lowest transmission ratios, the so-called Low-ratios, are usually primarily used during acceleration of a vehicle. In terms of efficiency it is thus important to be able to accurately control the line pressure when the transmission is in a OD-ratio, whereas a relatively inaccurate line pressure control in a Low-ratio will not have a large impact on the overall transmission efficiency. The present elaboration of the invention provides simple and cost efficient means to improve the accuracy of the control system particularly in OD-ratios. The factor with which the accuracy of the control system is improved in OD-ratios compared to the accuracy in Low-ratios is dependent on the design of the line pressure valve, but may typically be about 2 or even more.
According to yet another elaboration of the invention the pressure control means comprise a further valve capable of selectively allowing an essentially constant auxiliary pressure to serve as said further pressure. Preferably, said further valve is a switch-type valve, which allows either the control pressure or the auxiliary pressure to control the line pressure valve. Said auxiliary pressure is chosen somewhat higher than the maximum control pressure level that may be regulated by the pressure regulator. The line pressure valve is arranged such that under the influence of said auxiliary pressure the maximum applied line pressure level is set. According to the invention the pressure regulator feed pressure is particularly suited to serve as said auxiliary pressure. The feed pressure is used to set the line pressure valve when the applied line pressure lies in an upper most part of the range of applied line pressure levels, whereas the line pressure valve is controlled by the control pressure when said line pressure required for preventing belt slip lies in other parts of said range. According to the invention a threshold pressure PLP,th at which said upper most part starts can be calculated from:
PLP,th=PCP,max/PAUXxc2x7PLP,max
wherein: PCP,max is the maximum applied control pressure level,
PAUX is the auxiliary pressure, which in this case is equal to the pressure regulator feed pressure PPR,
PLP,max is the maximum applied line pressure level.
The advantage of this particular elaboration of the control system according to the invention is given by the insight that the higher line pressures are usually applied only when transmitting the highest torque levels, e.g. during a fast accelerations of a vehicle in which the CVT is adopted. It is found that during operation of the CVT such torque levels only occur in a relatively small part of the total time span of operation. This is particularly true if the CVT is provided with a torque converter, as is common practice in CVT design. It is thus possible to set the maximum line pressure level using the auxiliary pressure, even if a somewhat lower line pressure is actually required to prevent belt slip, while still maintaining good transmission efficiency. The entire range of control pressure levels is then available for control of the line pressure within only a part of the range of applied line pressure levels. The accuracy of the control system is thereby improved by a factor, which is given by the ratio between the auxiliary pressure and the maximum of the range of control pressure levels.
The invention also provides for a method allowing accurate control of the line pressure of the CVT. The method according to claim 13 is suited for the known CVT wherein the control system is capable of controlling the line pressure within a range of line pressure levels applied by means of a line pressure valve when operated by a control pressure varying-within a range of control pressure. In such a control system a control pressure resolution may be defined as the ratio between said range control pressure levels and a range of line pressure levels to be applied by means of a line pressure valve when operated by said range of control pressure levels. This resolution is a measure for the accuracy of the line pressure control. According the invention said resolution may be improved by providing for a further pressure capable of influencing the line pressure valve. In effect this means that several options are opened to improve the accuracy of the control system by decreasing said range of line pressure levels to be applied by means of a line pressure valve when operated by said range of control pressure levels. The method at least comprises the steps of:
determining a desired line pressure based on a number of signals at least representing the transmission ratio, the torque to be transmitted and the rotational speed of a pulley,
generating the control current for control of the pressure regulator at least in dependence on said desired line pressure and
providing a further pressure capable of controlling said line pressure valve so as to effect an increase of the control pressure resolution by decreasing the range of line pressure levels applied by means of a line pressure valve when operated by said range of control pressure levels.
According to a development of the invention a further pressure regulator for regulating a further control pressure which serves as said further pressure may be provided. In this case the method according to the invention additionally comprises the step of:
generating a further control current for control of a further pressure regulator at least in dependence on said desired line pressure, such that a further control pressure is regulated which serves as said further pressure. The particular advantages of this development of the invention being that the accuracy of the control system may be improved in the entire operational range of the CVT, e.g. independent on the transmission ratio, and that the improvement may be quite large, i.e. up to two a factor of 2.
According to another development of the invention a hydraulic line connecting the cylinder of the piston/cylinder assembly of the second pulley to the line pressure valve may be provided, such that the pressure in said cylinder serves as said further pressure. In this case the method according to the invention additionally comprises the step of:
determining a pressure actually present in the cylinder of the piston/cylinder assembly associated with the second pulley which serves as said further pressure Furthermore, in order to generate the appropriate control current for control of the pressure regulator, said current is generated additionally in dependence on the level of said pressure actually present in the cylinder of the piston/cylinder assembly associated with the second pulley. The particular advantage of this development of the invention being that the method may be implemented at low a cost.
According to still another development of the invention a valve may be provided for selectively allowing the line pressure valve to be controlled either by the control pressure or by an essentially constant auxiliary pressure which serves as said further pressure. In this case the method according to the invention additionally comprises the steps of:
determining if the desired line pressure is higher than a predetermined threshold pressure and
setting the line pressure valve by means of an essentially constant auxiliary pressure which serves as said further pressure if the desired line pressure is higher than said predetermined threshold pressure.
The particular advantage of this development of the invention being that the accuracy of the control system may be improved in the entire operational range of the CVT, e.g. independent on the transmission ratio.